Field of the Invention
Embodiments of the present invention relate, in general, to aerodynamic techniques designed to deter an aircraft's ability to enter a spin and more particularly to a configuration of said techniques that renders an aircraft spin resistant.
Relevant Background
The primary cause of fatal accidents in small aircraft is loss of control. Loss of control usually occurs because the aircraft enters a flight regime that is outside its normal envelope usually, but not always, at a high rate, thereby introducing an element of surprise for the flight crew. Factors leading to a loss of control are many including loss of situational awareness especially through distraction and/or complacency, intended or unintended mishandling of the aircraft, attempting to maneuver an aircraft outside its capabilities to resolve a prior problem, and the like. While every out of control situation is unique, one loss of control flight regime that every pilot it taught to avoid is a spin.
In aviation, a spin is an aggravated stall resulting in autorotation about a spin axis wherein the aircraft follows a corkscrew downward path. Stalls in fixed-wing flight are often experienced as a sudden reduction in lift as the pilot increases angle of attack and exceeds the critical angle of attack (which may be due to slowing down below stall speed in level flight). Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is a sufficient amount of yaw (rotation about a vertical axis) while an aircraft is stalled. In either case, however, a specific and often counterintuitive set of actions may be needed for an effective recovery to be made. If the aircraft exceeds published limitations regarding spins, or is loaded improperly, or if the pilot uses incorrect techniques to recover, the spin can, and often does, lead to a crash.
In a spin, both wings are in a stalled condition, but one wing will be in a deeper stall condition than the other. This causes the aircraft to autorotate (yaw) towards the deeper-stalled wing due to its higher drag. At the same time, the wings produce an unbalanced amount of lift causing the aircraft to roll and similarly adjust its pitch. An autorotation or spin is thus a stalled condition in which there is simultaneous movement around all three aircraft axes—that is, yaw, pitch, and roll. And as mentioned, a spin results in a vertical flight path. That is to say the aircraft is falling directly to the earth as it spins.
FIG. 1 is a high level depiction of the interaction of aerodynamic forces acting on a wing going into a spin as is known by one of reasonable skill in the relevant art. For discussion purposes, the wing 100 is divided into two portions designated by the direction the wing portion will go in a spin. In this particular illustration, there is a down-going wing portion 110 and an up-going wing portion 120. For an aircraft to spin, the wing must be stalled. In this example, the wing 100 is experiencing an angle of attack that is greater than the critical angle of attack and thus, a stalled condition results. Here both the down-going wing portion 110 and the up-going wing portion 120 are experiencing a stalled condition. However in this case, the down-going wing portion 110 possesses an angle of attack of 40 degrees 130 while the up-going wing portion 120 has an angle of attack of 25 degrees 135. The angle of attack of both wing portions 110, 120 exceeds the critical angle and are both stalled, yet the stall is not symmetrical. As the down-going wing portion 110 has a higher angle of attack 130, it will generate more drag 140 and less lift 150 than, comparatively, the drag 145 and the lift 155 of the up-going wing portion 120. The unbalanced forces result in the wing 100 yawing 160 and rolling 170 simultaneously.
Spins, typified by an excessive angle of attack and slow airspeed, differ from spiral dives which are characterized by low angle of attack and high airspeed. In a spiral dive, the airplane will respond conventionally to the pilot's inputs to the flight controls whereas in a spin, the aircraft's response to flight controls is compromised.
Some aircraft cannot be recovered from a spin using only their own flight control surfaces. Accordingly, if an aircraft has not been certified for spin recovery, it is assumed that spins are not recoverable and engaging in a spin is considered unsafe in that aircraft. For safety, all certificated, single-engine fixed-wing aircraft, including certificated gliders, must meet specified criteria regarding stall and spin behavior. Complying designs typically have a wing with greater angle of attack at the wing root (the part of the wing that is closest to the fuselage) than at the wing tip, so that the wing root stalls first, reducing the severity of the wing drop at the stall and possibly also allowing the ailerons to remain somewhat effective in controlling the aircraft's rolling movements until the stall migrates outward toward the wing tip. This ideally provides the pilot with some ability to control the aircraft in a stalled condition and preclude a spin from developing.
Beyond specifying criteria by which an aircraft must demonstrate certain stall and spin behavior, the Code of Federal Regulations that governs Aeronautics and Space, specifically 14 CFR § 23.221(a)(2), provides criteria by which an aircraft can be demonstrated to be “spin resistant.” Prior to the implementation of the present invention, no conventionally-configured aircraft has been able to successfully complete spin-resistance flight testing and demonstrate spin resistance in accordance with the 14 CFR § 23.221(a)(2) standard.
In the 1970s and 1980s, researchers at NASA's Langley Research Center studied spin resistance in depth, with a focus on aerodynamic characteristics and techniques to make aircraft more resistant to spins. They performed extensive modifications to several aircraft and flew thousands of test flights to determine how changes to the airframe would affect spin characteristics. What they discovered was that small changes could dramatically affect performance during spins. As a result of the study aircraft and according to NASA experimenters, aircraft should be designed to “give plenty of warning, lots of buffet, very little roll-off laterally—a long period of telling the pilot, ‘Hey, you're doing something wrong’.”
One of the key findings of the NASA studies was that a critical component of spin resistance is controlling the way the wing stalls. Experimenters concluded that having the stall initiate near the root of the wing while the outboard panels of the wing continue to fly is ideal because it prevents the stall from fully developing or “breaking” because the outboard panels are still generating lift. Without a stall, a spin cannot initiate.
Part 23, § 23.221 of 14 CFR requires that single-engine airplanes must demonstrate recovery from either a one-turn spin if intentional spins will be prohibited or six-turn spins if intentional spins will be approved. Even more advantageous than an aircraft able to recover from a spin is an aircraft that is resistant to spin entry. Despite decades of research and an understanding of the interaction between stalls and spins, the design of an aircraft meeting the standards of 14 CFR § 23.221 has remained a challenge. This and other obstacles of the prior art are addressed by one or more embodiments of the present invention. That is to say, the present invention provides an aircraft configuration that is spin resistant and in compliance with 14 CFR § 23.221(a)(2).
Additional advantages and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities, combinations, compositions, and methods particularly pointed out in the appended claims.